Responses of breeding Cory's shearwater Calonectris diomedea to experimental manipulation of chick condition

We studied the regulation of provisioning in Gory's shearwater at Selvagem Grande during the chick rearing period. Provisioning was examined in terms of feeding frequency and amount of food delivered to chicks. Two groups of chicks were subjected to short-term contrasting manipulations of their nutr itional status: one group of chicks was given a food supplement of about 30 g, and another group was deprived of up to 30 g of food. Adults tending de prived chicks increased the frequency of feeding visits (but not the size o f feeds), which resulted in an increase in the net rate of food delivery At the end of this study, deprived. chicks were growing at the same rate as f ed chicks. Parents attending fed chick did not change their provisioning ra tes in response to the treatment. Our results indicate that Gory's shearwat ers are able to adjust their provisioning rate in response to short-term va riation in the nutritional status of their chicks. We also examined the cha nge in the begging rate of fed and deprived chicks in response to the treat ment. There was no relationship between the begging rate and the condition of chicks, which is taken to be a measure: of the chick's physiological con dition, related to its ability to withstand imposed periods of fasting. How ever, fed chicks decreased their begging rate after the increase in their c ondition due to supplementary food. Conversely, deprived chicks, which were only able to sustain their condition before the onset of the treatment, ma intained high levels of begging. To some extent, these results suggest that parental provisioning can be influenced by the begging behavior of chicks.